1. Field of the Invention
The present invention relates to a signal transmission apparatus and a modulation method of an optical modem, and more particularly, to a method and apparatus for optimizing a modulation method of an optical modem in an orthogonal frequency division multiple access-passive optical network (OFDMA-PON).
2. Description of the Related Art
Currently, an intensity modulation/direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) system is attracting more and more attention as next-generation optical access network technology since the IM/DD optical OFDM system enables cost-effective optical transmission, effectively controls network resources, and also does not need any compensation technology for optical impairment in long-distance ultrahigh-speed transmission.
For the optical OFDM system, various adaptive loading algorithms have been being suggested, which measured a decrease of a signal to noise ratio experienced by each subcarrier in transmission media and enable each subcarrier to have a different bit and power level based on previously measured SNR (signal to noise ratio). Popular algorithms currently in use include a bit-loading algorithm, a power-loading algorithm, and a bit-and-power loading algorithm which is a combination of a bit-loading algorithm and a power-loading algorithm.
A bit-loading (BL) algorithm has the same electric power level for each subcarrier in the OFDM system but uses a modulation format mapped differently for the each subcarrier. Conversely, a power-loading (PL) algorithm provides the same modulation format for the each subcarrier in the OFDM system but has an electric power level set differently for the each subcarrier. Further, a combination of a bit-loading algorithm and a power-loading algorithm, a bit-and-power loading (BPL) algorithm, enables subcarriers to have different modulation formats and electric power levels, respectively, and maximizes transmission performance and a bandwidth in transmission systems.
Among three aforementioned algorithms, the BPL algorithm involves highly advanced digital signal processing technology and a very complicated design for an optical OFDM modem since it can optimize a transmission bandwidth and performance.
In a conventional OFDMA-PON system, in order for the entire system to identify frequency-response characteristics for the BL, PL, and BPL algorithms used to maximize a transmission performance, an OFDM transmission apparatus and an OFDM reception apparatus transmit and detect a pilot signal and analyze frequency-response characteristics of the entire system based on the pilot signal, an upper level, that is, a media access control (MAC) layer, controls a modulation method and an electric power level for each subcarrier corresponding to the frequency-response characteristics of the entire system, and a baseband modem maps and transmits a signal. In this method, however, not only a physical layer but also a MAC layer needs an additional function of analyzing and controlling the frequency-response characteristics, and the transmission apparatus and the reception apparatus may pair together to transmit, receive and analyze the pilot signal. That is, since a large number of elements with complicated functions require functions for analyzing the frequency-response characteristics, disadvantages on terms of cost and complexity may be present.
Further, when a logical/physical link is not established between the transmission apparatus and the reception apparatus, the loading algorithms may not be applied. Since the loading algorithms are designed for transmission characteristics of a wireless signal involving considerably changeable channel environments, application of the algorithms is substantially limited when relatively stable frequency-response characteristics are extracted as in a next-generation optical access network system generally using a single-mode optical fiber as a transmission media.